Land area devoted to sugarcane (Saccharum spp.) production in Brazil has increased from 2 million to 10 million ha over the past four decades. Studies have shown that, from an environmental perspective, the transformation of nitrogen (N) fertilizers into N 2 O gases can offset the advantages gained by replacing fossil fuels with biofuels. Our objectives here were to review recent developments in N management for sugarcane-biofuel production and assess estimates of N use efficiency (NUE) and N losses based on future scenarios, as well as for life-cycle assessments of bioenergy production. Approximately 60 % of N-based fertilizer applied to sugarcane fields in Brazil is recovered by plants and soils, whereas N losses to leaching and N 2 O emissions can average 5.6 and 1.84 % of the total applied N, respectively. Maintenance of trash, rotation with N-fixing legume species, and optimization of byproducts usage have potential for reducing the N requirements of sugarcane cultivation in Brazil. Moreover, the development of sugarcane genotypes with higher NUEs, along with management systems that consider soil capacity of mineralization, is required for improving the NUE of sugarcane. Strategies to maintain N as NH 4 + in sugarcane-cropped soils also have the potential to reduce N losses and enhance NUE. The development of secondgeneration biofuels is important for increasing biofuel production while simultaneously maintaining N rates and improving NUE, and sugarcane systems in Brazil show potential for sustainable biofuel production with low N rates and limited N 2 O losses. Reducing N rates in sugarcane fields is thus necessary for improving sugarcane-based biofuel production and reducing its environmental impacts.

BackgroundPlants are challenged by a large number of environmental stresses that reduce productivity and even cause death. Both chloroplasts and mitochondria produce reactive oxygen species under normal conditions; however, stress causes an imbalance in these species that leads to deviations from normal cellular conditions and a variety of toxic effects. Mitochondria have uncoupling proteins (UCPs) that uncouple electron transport from ATP synthesis. There is evidence that UCPs play a role in alleviating stress caused by reactive oxygen species overproduction. However, direct evidence that UCPs protect plants from abiotic stress is lacking.Methodology/Principal FindingsTolerances to salt and water deficit were analyzed in transgenic tobacco plants that overexpress a UCP (AtUCP1) from Arabidopsis thaliana. Seeds of AtUCP1 transgenic lines germinated faster, and adult plants showed better responses to drought and salt stress than wild-type (WT) plants. These phenotypes correlated with increased water retention and higher gas exchange parameters in transgenic plants that overexpress AtUCP1. WT plants exhibited increased respiration under stress, while transgenic plants were only slightly affected. Furthermore, the transgenic plants showed reduced accumulation of hydrogen peroxide in stressed leaves compared with WT plants.Conclusions/SignificanceHigher levels of AtUCP1 improved tolerance to multiple abiotic stresses, and this protection was correlated with lower oxidative stress. Our data support previous assumptions that UCPs reduce the imbalance of reactive oxygen species. Our data also suggest that UCPs may play a role in stomatal closure, which agrees with other evidence of a direct relationship between these proteins and photosynthesis. Manipulation of the UCP protein expression in mitochondria is a new avenue for crop improvement and may lead to crops with greater tolerance for challenging environmental conditions.

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BackgroundDrought is a major abiotic stress that affects crop productivity worldwide. Sugarcane can withstand periods of water scarcity during the final stage of culm maturation, during which sucrose accumulation occurs. Meanwhile, prolonged periods of drought can cause severe plant losses.Methodology/Principal FindingsIn a previous study, we evaluated the transcriptome of drought-stressed plants to better understand sugarcane responses to drought. Among the up-regulated genes was Scdr1 (sugarcane drought-responsive 1). The aim of the research reported here was to characterize this gene. Scdr1 encodes a putative protein containing 248 amino acids with a large number of proline (19%) and cysteine (13%) residues. Phylogenetic analysis showed that ScDR1is in a clade with homologs from other monocotyledonous plants, separate from those of dicotyledonous plants. The expression of Scdr1 in different varieties of sugarcane plants has not shown a clear association with drought tolerance.Conclusions/SignificanceThe overexpression of Scdr1 in transgenic tobacco plants increased their tolerance to drought, salinity and oxidative stress, as demonstrated by increased photosynthesis, water content, biomass, germination rate, chlorophyll content and reduced accumulation of ROS. Physiological parameters, such as transpiration rate (E), net photosynthesis (A), stomatal conductance (gs) and internal leaf CO2 concentration, were less affected by abiotic stresses in transgenic Scdr1 plants compared with wild-type plants. Overall, our results indicated that Scdr1 conferred tolerance to multiple abiotic stresses, highlighting the potential of this gene for biotechnological applications.

Current limitations on the use of conventional N fertilizers on sugarcane (Saccharum spp.) in Brazil require the search for alternative sources with adequate N use efficiency. Field experiments were conducted in Typic Hapludox (TH) and Typic Eutrustox (TE) soils in São Paulo State, Brazil, to evaluate the dynamics of soil mineral N, N uptake, and sugarcane yield in response to N amendments including Ajifer‐8, ammonium chloride, ammonium nitrate, calcium ammonium nitrate (CAN), and urea applied on sugarcane ratoon at 100 kg N ha−1. The experiments were arranged in randomized complete block design, with four replications. The treatments were applied at 105 and 122 days after harvest of plant cane (DAH) in the TH and TE soil. Ajifer‐8 exhibited higher NH4+–N availability in the 0‐ to 20‐cm soil layer 139 DAH in the TH soil. Ammonium nitrate and CAN resulted in the highest NO3−–N values in the soil within the same period. In the TE soil, the soil mineral N content remained unaltered during all of the sampling dates. The high values of accumulated N uptake by sugarcane in both soils indicates that a good portion of the soil mineral N was take up by the plants. The Ajifer‐8 and CAN fertilizers resulted in higher sugarcane yield in both experiments, and may be used as replacement for conventional N sources. In contrast, the use of ammonium chloride resulted in low values of soil mineral N and accumulated N uptake.

Knowing the right fertilizer N source is a key management practice for improving nutrient use and productivity for sugarcane (Saccharum spp.). Therefore, this study evaluated the effect of fertilizer N sources on aboveground biomass (BM) and nutrient (N, P, K, Ca, Mg, and S) content for sugarcane ratoon. Two field trials under highly weathered tropical soils (Site 1, Typic Hapludox; Site 2, Typic Eutrustox) were conducted in the southeast region of Brazil. Five fertilizer N sources (organomineral fertilizer [OMF], ammonium chloride [AC], ammonium nitrate [AN], calcium ammonium nitrate [CAN], and urea) were applied on sugarcane at a rate of 100 kg N ha−1. A control treatment was also included in the randomized complete block design. At harvest, total BM varied from 24.4 Mg ha−1 (control and AC) at Site 1 to 44.2 Mg ha−1 (CAN) at Site 2, total K content varied from 185 kg ha−1 (AC) at Site 2 to 747 kg ha−1 (OMF) at Site 1, and total N content ranged from 92 kg ha−1 (AC) at Site 2 to 165 kg ha−1 (AN) at Site 1. Use of CAN and OMF resulted in comparable or superior BM and nutrient content relative to the other N fertilizers. In contrast, AC did not increase BM and nutrient content (except for N and Mg at Site 1), but also decreased by 33% total N content than control at Site 2. Use of more efficient fertilizer N sources (e.g., CAN and OMF) could increase productivity and nutrient use for sugarcane ratoon systems.

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Numerous plant species can release organic acid anions (OA) from their roots in response to toxic aluminium (Al) ions present in the rooting medium. Hypothetically OA complex Al in the root apoplast and/or rhizosphere and thus avoid its interaction with root cellular components and its entry in the root symplast. Two temporal patterns of root OA exudation are observed. In pattern I, OA release is rapidly activated after the contact of the root with Al ions while in pattern II there is a lag phase between the addition of Al and the beginning of OA release. Compounds other than OA have been detected in root exudates and are also correlated with Al resistance in plants. Plant species like buckwheat and tea show mechanisms of Al tolerance, which confer them the capacity to inactivate and store Al internally in the leaves. Disturbances in metabolic pathways induced by Al are still obscure and their relation to the altered OA concentration observed in roots under Al stress is not yet established. High concentrations of OA in roots do not always lead to high rates of OA release even when the spatial distribution of these two characteristics along the root axis is taken into account. Al induces high permeability to OA in young root cells and anion channels located in the cell membrane have been proposed to mediate the transport of OA to outside the cell. Genetically modified plants that overexpress genes involved in the biosynthesis and transport of OA as well as in Al toxicity events at the cell level have been generated. In most cases the transformations resulted in an improved ability of the plant to cope with Al stress. These promising findings reinforce the possibility of engineering plants with superior resistance to Al-toxic acid soils. The environmental impact of the large amounts of root exudates possibly conferred by these genetically modified plants is discussed, with special emphasis on soil microbiota.